bims-mecosi 2024-12-08 papers

Issue of 2024–12–08
six papers selected by
Verena Kohler, Umeå University

Am J Physiol Cell Physiol. 2024 Dec 02.

STIM1 and lipid interactions at ER-PM contact sites.

Yuepeng Ke, Ritchel Gannaban, Junchen Liu, Yubin Zhou.

Store-operated calcium (Ca2+) entry (SOCE) represents a major route of Ca2+ permeation across the plasma membrane (PM) in non-excitable cells, which plays an indispensable role in maintaining intracellular Ca2+ homeostasis. This process is orchestrated through the dynamic coupling between the endoplasmic reticulum (ER)-localized Ca2+ sensor stromal interaction molecule 1 (STIM1) and the PM-resident ORAI1 channel. Upon depletion of ER Ca2+ stores, STIM1 undergoes conformational rearrangements and oligomerization, leading to translocation of STIM1-containing ER membrane towards the PM. This movement is facilitated by the physical interaction between positively charged cytosolic domains within STIM1 and negatively charged phospholipids embedded in the PM, ultimately enabling its binding to and activation of the PM-embedded ORAI1 channel. In this mini-review, we provide an overview of STIM1-mediated Ca2+ signaling at ER-PM contact sites, highlighting the regulatory roles of phospholipids in the inner leaflet and sphingolipids in the outer leaflet of the PM. We also discuss the development of molecular tools that enable real-time visualization and manipulation of membrane contact sites (MCSs) at ER-PM junctions. Lastly, we highlight recent progress in developing targeted therapies for human diseases linked to STIM1 mutations and dysregulated Ca2+ signaling at ER-PM MCSs.

Keywords: Calcium release-activated calcium channel (CRAC); Calcium signaling; membrane contact sites (MCS); phosphoinositide signaling; stromal interacting molecule 1 (STIM1)

DOI: https://doi.org/10.1152/ajpcell.00634.2024

PLoS Pathog. 2024 Dec 02. 20(12): e1012635

Molecular characterization of the permanent outer-inner membrane contact site of the mitochondrial genome segregation complex in trypanosomes.

Philip Stettler, Bernd Schimanski, Salome Aeschlimann, André Schneider.

The parasitic protozoan Trypanosoma brucei has a single unit mitochondrial genome linked to the basal body of the flagellum via the tripartite attachment complex (TAC). The TAC is crucial for mitochondrial genome segregation during cytokinesis. At the core of the TAC, the outer membrane protein TAC60 binds to the inner membrane protein p166, forming a permanent contact site between the two membranes. Although contact sites between mitochondrial membranes are common and serve various functions, their molecular architecture remains largely unknown. This study elucidates the interaction interface of the TAC60-p166 contact site. Using in silico, in vitro, and mutational in vivo analyses, we identified minimal binding segments between TAC60 and p166. The p166 binding site in TAC60 consists of a short kinked α-helix that interacts with the C-terminal α-helix of p166. Despite the presence of conserved charged residues in either protein, electrostatic interactions are not necessary for contact site formation. Instead, the TAC60-p166 interaction is driven by the hydrophobic effect, as converting conserved hydrophobic residues in either protein to hydrophilic amino acids disrupts the contact site.

DOI: https://doi.org/10.1371/journal.ppat.1012635

Mol Biol Cell. 2024 Dec 04. mbcE24040197

Movement of the endoplasmic reticulum is driven by multiple classes of vesicles marked by Rab-GTPases.

Allison Langley, Sarah Abeling-Wang, Erinn Wagner, John Salogiannis.

Peripheral endoplasmic reticulum (ER) tubules move along microtubules to interact with various organelles through membrane contact sites (MCS). Traditionally, ER moves by either sliding along stable microtubules via molecular motors or attaching to the plus ends of dynamic microtubules through tip attachment complexes (TAC). A recently discovered third process, hitchhiking, involves motile vesicles pulling ER tubules along microtubules. Previous research showed that ER hitchhikes on Rab5- and Rab7-marked endosomes, but it is uncertain if other Rab-vesicles can do the same. In U2OS cells, we screened Rabs for their ability to cotransport with ER tubules and found that ER hitchhikes on post-Golgi vesicles marked by Rab6 (isoforms a and b). Rab6-ER hitchhiking occurs independently of ER-endolysosome contacts and TAC-mediated ER movement. Depleting Rab6 and the motility of Rab6-vesicles reduces overall ER movement. Conversely, relocating these vesicles to the cell periphery causes peripheral ER accumulation, indicating that Rab6-vesicle motility is crucial for a subset of ER movements. Proximal post-Golgi vesicles marked by TGN46 are involved in Rab6-ER hitchhiking, while late Golgi vesicles (Rabs 8/10/11/13/14) are not essential for ER movement. Our further analysis finds that ER to Golgi vesicles marked by Rab1 are also capable of driving a subset of ER movements. Taken together, our findings suggest that ER hitchhiking on Rab-vesicles is a significant mode of ER movement. [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text].

DOI: https://doi.org/10.1091/mbc.E24-04-0197

J Microsc. 2024 Dec 04.

The photosensitive endoplasmic reticulum-chloroplast contact site.

Sara N Maynard, Lawrence R Griffing.

The endoplasmic reticulum (ER) forms contact sites with the chloroplast. Exposing contact sites that contain both the chloroplast and the ER to localised high-fluence, wavelength specific, 405 nm violet light, hereinafter referred to as photostimulation, induces multiple, potentially interacting intra- and intercellular responses. The responses vary depending on the tissue type of the cell and the chloroplast. Photostimulating the ER-chloroplast contact sites in growing epidermal cells of the hypocotyl of Arabidopsis thaliana, produces a wave of cytoplasmic ionic calcium that traverses the cell, spreading radially to other cells around the circumference of the hypocotyl. A transient ER stress accompanies the calcium wave. These responses occur in older epidermal cells (5-8 days post-germination) with nonmotile chloroplasts tethered to the ER and the cell cortex but do not occur with motile or dividing chloroplasts. Dividing chloroplasts show a markedly different association with the ER, which forms a ring around the fission plane, similar to that of dividing mitochondria. Inhibition of calcium channels with lanthanum has no effect. Photostimulation of only the ER results in no ER stress and a calcium wave with a different spatiotemporal signature: delayed release and lower magnitude, with no accompanying ER stress response. Likewise, photostimulation of the chloroplast only, without the ER, produces no calcium wave or ER stress. General chloroplast photobleaching or restructuring caused by photostimulation is not the cause of this response; photostimulation with 488 nm of the same intensity and power as 405 nm photostimulation produces no change in cytosolic calcium levels. The pH of the ER decreases, indicating the involvement of ER ion transporters in the response. A wave of increased reactive oxygen species (ROS) in mitochondria and nuclei accompanies photostimulation. Together, these data support a model by which tethered ER-chloroplast contact sites constitute a unique subcellular photosensitive region and are part of an ER-mediated signalling network. Lay Abstract: The endoplasmic reticulum (ER) forms contact sites with the chloroplast. Shining violet (405 nm) light on the chloroplast with its associated ER produces a calcium wave through the cell that is communicated to other cells. This is correlated with a wave of transient denaturation of the luminal proteins of the ER (ER stress) and increased reactive oxygen species (ROS) in mitochondria. The wavelength dependence and precise cellular location of the light stimulation implies a novel way for plants to sense light. The movement of the response through the cell is consistent with the mediation of the response by a subcellular network, such as that formed by the ER.

Keywords: calcium wave; endoplasmic reticulum pH; endoplasmic reticulum stress response; near‐UV photosensor; reactive oxygen species wave

DOI: https://doi.org/10.1111/jmi.13377

Front Cell Neurosci. 2024 ;18 1470144

MAM-mediated mitophagy and endoplasmic reticulum stress: the hidden regulators of ischemic stroke.

Ziyi Jia, Hongtao Li, Ke Xu, Ruobing Li, Siyu Yang, Long Chen, Qianwen Zhang, Shulin Li, Xiaowei Sun.

Ischemic stroke (IS) is the predominant subtype of stroke and a leading contributor to global mortality. The mitochondrial-associated endoplasmic reticulum membrane (MAM) is a specialized region that facilitates communication between the endoplasmic reticulum and mitochondria, and has been extensively investigated in the context of neurodegenerative diseases. Nevertheless, its precise involvement in IS remains elusive. This literature review elucidates the intricate involvement of MAM in mitophagy and endoplasmic reticulum stress during IS. PINK1, FUNDC1, Beclin1, and Mfn2 are highly concentrated in the MAM and play a crucial role in regulating mitochondrial autophagy. GRP78, IRE1, PERK, and Sig-1R participate in the unfolded protein response (UPR) within the MAM, regulating endoplasmic reticulum stress during IS. Hence, the diverse molecules on MAM operate independently and interact with each other, collectively contributing to the pathogenesis of IS as the covert orchestrator.

Keywords: endoplasmic reticulum stress; ischemic stroke (IS); mitochondrial-associated endoplasmic reticulum membrane (MAM); mitophagy; unfolded protein response (UPR)

DOI: https://doi.org/10.3389/fncel.2024.1470144

Exp Mol Med. 2024 Dec 03.

The anticancer effect of metformin targets VDAC1 via ER-mitochondria interactions-mediated autophagy in HCC.

Minjeong Ko, Jiho Kim, Raudah Lazim, Ju Yeon Lee, Jin Young Kim, Vijayakumar Gosu, Yoonji Lee, Sun Choi, Ho Jeong Kwon.

Metformin (MetF) is used worldwide as a first-line therapy for type 2 diabetes. Recently, interest in the pleiotropic effects of MetF, such as its anticancer and antiaging properties, has increased. However, the molecular target of MetF and the detailed mechanism underlying its ability to inhibit cell growth through autophagy induction remain incompletely understood. In this study, using an innovative label-free drug affinity responsive target stability (DARTS)-LC-MS/MS method, we discovered that mitochondrial voltage-dependent anion channel 1 (VDAC1) is a novel binding protein involved in the induction of autophagy-related cell death by high-dose MetF in hepatocellular carcinoma (HCC). Computational alanine scanning mutagenesis revealed that MetF and VDAC1 (D9, E203) interact electrostatically. MetF disrupts the IP3R-GRP75-VDAC1 complex, which plays a key role in stabilizing mitochondria-associated ER membranes (MAMs), by binding to VDAC1. This disruption leads to increased cytosolic calcium levels, thereby contributing to autophagy induction. MetF also decreased the AMP/ATP ratio and activated the AMPK pathway. Cells with genetic knockdown of VDAC1 mimicked the activity of MetF. In conclusion, this study provides new insights into the involvement of MetF in ionic interactions with VDAC1, contributing to its anticancer effects in HCC. These findings help elucidate the diverse biological and pharmacological effects of MetF, particularly its influence on autophagy, as well as the potential of MetF as a therapeutic agent for diseases characterized by VDAC1 overexpression.

DOI: https://doi.org/10.1038/s12276-024-01357-1